When John Wayne played Genghis Khan in the 1956 film, “The Conquerer,” he quickly secured his place on every “Worst Films of all Time” list for the next 50 years, as his portrayal was so wooden and wrong, and his casting so ill-advised, that the film became a legendary cautionary tale against stunt casting.

But the biggest disaster that eventually engulfed “The Conquerer” had nothing to do with the acting.

Of the film’s 220 cast and crew members, 91 eventually developed cancer. In a traditional sample of that size, the number of lifetime cancer cases would normally range between 30-40.

The posited reason for this outbreak — which eventually killed Wayne, co-stars Susan Hayward and Agnes Moorehead, and the film’s director, Dick Powell — was the close proximity of the film’s Utah location to the Nevada site where, just one year earlier, the US Army conducted “one of the dirtiest atmospheric detonations of all time,” says Rudolph Herzog in his upcoming book “A Short History of Nuclear Folly” (Melville House).

Herzog, son of acclaimed filmmaker Werner Herzog, writes that the test released a radioactive cloud that “ascended to an elevation of 11,600 feet before topping out and heading east,” when it showered unsuspecting civilians in radiation up to 170 miles from the test site.

“At least a third of all radiation released by all American nuclear tests between 1951 and 1958,” writes Herzog, “came from this one exercise.”

What’s terrifying is that the 1955 mishap is just one example of the remarkably thoughtless and careless handling of nuclear weaponry by the world’s governments. If you think Kim Jong Un is reckless with his nukes, consider the early days of the Cold War.

In 1953, the Soviets blew up radioactive bombs on the surface of Europe’s largest inland lake, Lake Ladoga, which supplied drinking water to St. Petersburg.

In 1969, the US set off a one megaton hydrogen bomb on the Alaskan Island of Amchitka that raised the island’s surface 15 feet, while causing other widespread damage, including earthquakes.

Herzog — who notes that “30 to 40 atomic and hydrogen bombs have officially disappeared, though some experts speculate that the number may be as high as 50” — outlines several incidences where the US or the Soviets lost, dropped or accidentally launched nuclear weapons for no reason other than “oops.”

“There were dozens of accidents,” he writes, “in which nuclear bombs were damaged or even lost during the Cold War — in both the West and the East. In military parlance, these were known as ‘broken arrows.’ ”

On the Soviet side, Herzog cites one rumor that “a Soviet submarine laid 20 nuclear mines on the ocean floor in the Bay of Naples,” and, in a more well-documented incident, says that a Soviet submarine “sank in the Pacific in 1968 . . . carrying several nuclear missiles to be fired at the West Coast of the United States in the event of an all-out war.”

(The US embarked on a costly and controversial project to recover that sub and its cargo. Herzog says the exact results of this are unknown, but that evidence points to a partial recovery, with the US bringing part of the sub, and several nuclear torpedoes, to the surface, while other nuclear torpedoes remain at the bottom of the ocean to this day.)

Herzog also outlines several better-detailed instances where the US came frighteningly close to bombing unplanned targets, including a few right here at home.